Surface treatment techniques, which provide various excellent functions on a surface to be treated of a substrate, by performing a surface treatment on the surface of the substrate, are a technology which attracts attentions. Although there are various methods for surface treatment, the method of performing surface treatment comprising using graft polymerization accompanied by a covalent bond by photo-polymerization is widely used in recent years in broad fields, such as plastic molded product materials, plastic film materials, optical lens materials, medical materials, and magnetic materials.
This type of graft polymerization can be classified broadly into two major categories; (1) a surface-initiated graft polymerization, so-called a “grafting from” method and (2) a graft polymerization using the adsorption of the polymers (dip coating, crosslinking, binding by the reactive functional group in the polymers), so-called a “grafting to” method. In order to provide various excellent functions on a surface which was treated by graft polymerization by using monomers having various functions, the “grafting from” method which is the surface-initiated type graft polymerization is an effective method.
As one method of performing graft polymerization, a photo-graft polymerization method, wherein the graft polymerization reaction is initiated by irradiation of a light (electromagnetic wave) having a predetermined wavelength, is continuing to progress along with the development of a photo-polymerization initiator and the improvement of an irradiation apparatus to be used. The applications of the photo-graft polymerization method are expected to have a new progress in the future in the fields such as an information industry, an automobile/precision instruments industry, and a medical industry.
Conventionally, as the photo-polymerization initiator of the photo-graft polymerization, a photo-radical initiator is mainly used. It is necessary to apply a radical initiator beforehand to the surface to be treated (for example, the surface of a substrate which is to be treated) for performing the “grafting from” method.
In one example of the conventional photo-graft polymerization method, a film or layer of 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer is formed by graft polymerizing MPC onto the surface of a substrate by using polyethylene (PE) as the substrate, MPC as a reactive monomer, and benzophenone (BP) as a photo-polymerization initiator (Patent Documents 1 and 2).
Alternatively, it is proposed that a simultaneous irradiation method which comprises irradiating simultaneously a substrate and a monomer with radiation for the graft polymerization, or a prior irradiation method comprising irradiating the substrate with a gamma ray, followed by contacting the substrate with the monomer, thereby performing a graft polymerization (Patent Document 3).    [Patent Document 1] JP-A-2003-310649    [Patent Document 2] JP-A-2007-202965    [Patent Document 3] JP-A-2008-53041
The method of Patent Document 1 is shown by the following scheme 1:

In the method of the above-mentioned Patent Document 1, in order to uniformly work the radical initiator as the polymerization initiator onto the surface of the substrate and to uniformly coat the given surface of the substrate with the polymerization product, it is necessary to dissolve a radical initiator in a solvent and then to apply the radical initiator solution to the surface of the substrate. However, there was a problem that the solvent exerts unintended functions, such as erosion or dissolution action on the surface of the substrate to be treated, depending on types of the solvent.
Further, in order to realize the surface-initiated graft polymerization, it is necessary to select such a solvent that the monomers are soluble in the solvent, while the radical initiator is insoluble or hardly-soluble in the solvent, as the property of the solvent to be used for the photo-graft polymerization. Therefore, there was the problem that the options are limited.
The MPC copolymer produced by the method of Patent Document 1 is useful as the material for forming an ideal biocompatible surface. However, in the case where the product therefrom is used as a biocompatible material, it is desirable that no radical initiator remains on the surface of the substrate and in the graft polymer layer after performing the graft polymerization reaction. Therefore, in the method shown in the above Scheme 1, there was another problem of removing the radical initiator that remains after performing the graft polymerization reaction from the surface of the substrate and the graft polymer layer.
The method shown by Patent Document 3 uses high energy radiations, for example, gamma rays, electron beams (beta rays), ion beam, and X-rays in order to form a free radical. These radiations themselves have so high risks that large-scale facilities are further required for controlling the radiation source, therefore there are problems with respect to safety and economical efficiency.
According to the simultaneous irradiation method proposed by Patent Document 3 as the specific method of graft polymerization, a sufficient graft density (for example, at least 0.01 chains/nm2) is not obtained by the graft polymerization, since the free radicals are generated from both of the substrate and the monomer. In addition, there were the problems that unnecessary or undesirable molecular cleavage is caused, since the radiation such as gamma rays can pass through the substrate, thereby giving deterioration or embrittlement of the substrate. The graft density is described, for example, in “New Frontiers in Polymer Synthesis” Advances in Polymer Science, Vol. 217, 2008.
According to the pre-irradiating method, when the period of time, after irradiation until the substrate on which the free radicals being generated is contacted with the monomers, is elongated, the available free radical decreases with passage of time, thereby it is capable of failing to obtain a desirable and sufficient graft rate (density). The problem of deterioration (or embrittlement) of the substrate accompanied by irradiating with radiation, such as gamma rays still remains, as in the case of simultaneous irradiation method.